From the scaling analysis of chain dynamics in the last chapter, the fluid
transitionTfcan be understood as the temperature corresponding to the characteris-
tic reptation time. AtTf, the whole chain can diffuse out of the tube formed by the
dynamic entanglement of chains via spontaneous Brownian motions. So this tran-
sition is indeed a dynamic relaxation transition. According to the Ueberreiter’s
judgment for the extrapolation of the fluid transition to small molecules, glass
transition can be regarded as the temperature for molecular particles to be able to
diffuse out of a dynamic sticking network via spontaneous Brownian motions,
which in nature is a dynamic relaxation transition as well. Recently, it has been
believed that belowTf,abbifurcation is the origin of VFT-type non-Debye
relaxation of polymer fluids, and the same behaviors exist also belowTmfor
semi-crystalline polymers (Rault 2000 ). Therefore,arelaxation is a cooperative
movement, whilebrelaxation is a non-cooperative movement.
6.4 Conventional Mechanical Analysis
The discussions above focus on the small strain as a response of polymer materials to
the small stress. Large stress brings large strain and even destroys the inherent
structure of the solid materials, causing permanent deformation. Under the constant
strain rates, the stress–strain curve reflects the structural and viscoelastic characteris-
tic features of materials. For polymer materials, there occur five typical curves, as
illustrated in Fig.6.18: (1) hard and brittle, such as PS and PMMA, eventually brittle
failure; (2) hard and tough, such as Nylon and PC, most of semi-crystalline polymers,
Fig. 6.18Illustration of five conventional stress–strain curves of polymer materials under con-
stant strain rates. 1 hard and brittle, 2 hard and tough, 3 hard and strong, 4 soft and tough, and 5 soft
and weak
6.4 Conventional Mechanical Analysis 119